1. Field of the Invention
The present invention relates generally to a method and apparatus for controlling power in a photovoltaic system, and more particularly, to a power control method and apparatus for tracking a maximum power point in the photovoltaic system
2. Description of the Related Art
With the depletion of natural resources and the emerging environmental and safety issues surrounding existing power generation methods such as thermal power generation and nuclear power generation, many studies are being conducted on photovoltaic power generation, which is an example of a renewable energy source.
Photovoltaic power generation, a clean energy source that does emit pollutants that are harmful to the global environment, such as greenhouse gases, may be applied to residential power systems and vehicle power systems, and also to various other fields such as streetlights, lighthouses, and communication devices.
When a photovoltaic power generation system (or a photovoltaic system for short) generates its maximum power, the intersection of power and voltage on a power-voltage (P-V) characteristic curve of a solar cell is called a Maximum Power Point (MPP). Power generated by the photovoltaic system may be subject to change depending on the surrounding environment such as solar intensity, temperature, and cloud coverage.
Therefore, it is important for the photovoltaic system to maintain the system operating point (i.e., operation voltage of its solar cells) at an MPP, where output powers of the solar cells are highest, by tracking the MPP to make it possible to generate the maximum power in the time-varying surrounding environment. To maintain the system operating point at an MPP, a variety of algorithms have been proposed, which will be referred to as Maximum Power Point Tracking (MPPT) algorithms.
FIG. 1 is a graph illustrating power-voltage (P-V) characteristics of a conventional MPPT algorithm.
Referring to FIG. 1, on a P-V characteristic curve of a solar cell or photovoltaic (PV) array, a PV array voltage at an MPP 101 is Vmax. Further, a voltage slope and a power slope are directly proportional in a range of a PV array voltage lower than the PV array voltage Vmax, and the voltage slope and power slope are inversely proportional in a range of a PV array voltage higher than the PV array voltage Vmax. Based on these characteristics, digital control devices may measure an infinitesimal fluctuation between the discrete current value and previous value of a power value, and determine a switching operation for MPPT. A Perturbation and Observation (P&O) scheme, which is a typical example of the MPPT algorithm, is often used because it has a simple feedback structure and a small number of measurement parameters.
FIG. 2 illustrates a P&O scheme according to a conventional MPPT algorithm. Specifically, the P&O scheme operates by periodically increasing and decreasing an operating voltage of a solar cell, and tracking and finding the MPP by comparing a previous output power with a current output power of the solar cell during the disturbance period.
Referring to FIG. 2, in step 201, an output power of a solar cell is measured. The power measurement may be performed at stated periods. In step 203, a current power measured in the current period is compared with a previous power measured in the previous period. If the current power is less than the previous power, a terminal voltage (or current voltage) of the solar cell in the current period is compared with a terminal voltage (or previous voltage) of the solar cell in the previous period in step 205. If the current voltage is greater than the previous voltage, the terminal voltage of the solar cell is gradually decreased by a predetermined increment in step 207. Step 207 is performed to approximate a power point of a point B3 to the MPP 101, when the power point has moved, for example, from a point B2 to the point B3 on the P-V characteristic curve illustrated in FIG. 1.
However, if the current voltage is less than the previous voltage in step 205, the terminal voltage of the solar cell is gradually increased by a predetermined increment in step 209. Step 209 is performed to approximate a power point of the point B2 to the MPP 101, when the power point has moved, for example, from a point B4 to the point B2 on the P-V characteristic curve illustrated in FIG. 1.
If the current power is greater than the previous power in step 203, the current voltage of the solar cell is compared with the previous voltage of the solar cell in step 211. If the current voltage is less than the previous voltage, the terminal voltage of the solar cell is gradually decreased by a predetermined increment in step 213. Step 213 is performed to approximate a power point of the point B4 to the MPP 101, when the power point has moved, for example, from the point B3 to the point B4 on the P-V characteristic curve illustrated in FIG. 1.
However, if the current voltage is greater than the previous voltage in step 211, the terminal voltage of the solar cell is gradually increased by a predetermined increment in step 215. Step 215 is performed to approximate a power point of the point B2 to the MPP 101, when the power point has moved, for example, from the point B1 to the point B2 on the P-V characteristic curve illustrated in FIG. 1.
If an output power of a solar cell increases, the disturbance will continuously increase in the same direction during the next period, and otherwise, the disturbance direction will be reversed, meaning that the voltage of the solar cell is disturbed over all MPPT periods. Therefore, even though solar radiation is constant when the output power of the solar cell has reached the MPP, the terminal voltage of the solar cell may undergo self oscillation by the P&O scheme, resulting in a reduction in the efficiency of the photovoltaic system.
In order to minimize the efficiency reduction, the P&O scheme may consider reducing the unit amount or the disturbance increment by which the voltage of the solar cell is increased and/or decreased gradually. In this case, however, a sudden change in the solar radiation may cause an increase in the latency in tracking and finding the MPP.